Recently, environmentally friendly vehicles mounting electric motor as a power source for moving the vehicles forward, such as electric vehicles and hybrid vehicles, come to be popular.
As an example of such a vehicle, Japanese Patent Laying-Open No. 2002-10670 discloses a vehicle in which a plurality of electric motors are driven independently using low DC-voltage to output power and to enhance efficiency of the device.
A hybrid vehicle that adopts a structure having large battery capacity and allowing charging from the outside, cuts operation rates of engine and requires less refueling has been studied. Such a vehicle will be referred to as an externally rechargeable hybrid vehicle.
An externally rechargeable hybrid vehicle is adapted to have a battery of higher output and larger capacity than that of an ordinary hybrid vehicle that allows replenishment of fuel only, to increase the range of EV running, whereby the vehicle predominantly runs as EV as long as the battery charge allows, attaining better mileage and lower emission of carbon dioxide.
By way of example, assume that one drives the externally rechargeable hybrid vehicle to and from work, for a relatively short distance. If charge is performed every night at home, the engine operates only when the state of charge of the battery comes close to zero after a long drive at the weekend, or when he/she steps the accelerator pedal and the vehicle burden increases considerably.
In order to improve the effect of reducing carbon dioxide emission in the externally rechargeable hybrid vehicle, it is necessary to use battery power with higher priority than in a common hybrid vehicle. A parallel type hybrid system or a series/parallel hybrid system, in which engine power can be split and transmitted to the axle and generator by a power split device, however, operates on the premise that engine torque is also used as driving torque at the time of abrupt acceleration or driving at high speed. Therefore, in an externally rechargeable hybrid vehicle, in order to attain comparable maximum driving torque without using engine torque and to enlarge EV running range, it is necessary to enlarge the size of motor and its driving circuitry to realize enhanced performance.
From the viewpoint of energy efficiency and manufacturing cost, however, larger size of driving circuitry and increased number of components should desirably be avoided.